1. Field of the Invention
This invention relates to an energy monitoring system for the supervision of a particle accelerator, preferably of a linear accelerator. Particularly, this invention relates to an X-ray energy interlock system for an electron linear accelerator of the type having no electron beam bending system which may act as an electron energy band pass.
2. Description of the Prior Art
It is known in the art of radiation systems of various types to switch off the radiation beam via an ionization chamber to which the radiation is applied, as soon as a previously determined dosage of radiation has been reached. Particularly in the case of particle accelerators, such as linear accelerators, it is known to use monitoring systems which control dosage and dosage rate during treatment and which ensure automatic termination of radiation if preset values are exceeded (see brochure "Mevatron 20" by Siemens AG, West Germany, Order No. MT 3/1702.101-WS 5791, particularly see page 9). Such safety interlock systems may be applied in linear accelerators in which the dose rate is uniformly fixed for X-ray irradiation of a single energy, such as to a value of 300 rad/min in the flattened field at 100 cm FD (see brochure "Mevatron 20," supra), or in linear accelerators in which the dose rate is continuously variable between a lower and an upper limit (see brochure "Mevatron 60, Data" by Siemens AG, West Germany, Order No. MT3-6027.101 -PA 9783). Such linear accelerators contain a target to generate X-rays and an X-ray flattening filter in the form of a cone shaped metal piece.
From U.S. Pat. No. 4,115,830 is known a monitoring system for the high-voltage supply of an ionization chamber. This system is preferably used for monitoring a particle accelerator. In the field of particle accelerators, it is known to regulate the radiation intensity or radiation output via the ionization current of an ionization chamber subjected to the radiation in such a way that the number of radiation pulses per time unit is changed in correspondence with the chamber signal measured. To overcome inaccuracies in the ionization current measurement below a minimum value of the high voltage supplied to the chamber, the monitoring system is provided. The monitoring system comprises a switch member which is associated with a safety circuit of the particle accelerator and which switches off the latter in the event of insufficient high-voltage.
It is also known in an accelerator to use an interlock system that automatically interlocks the machine according to a signal which represents the homogeneity and/or symmetry of the radiation beam (see, for instance, brochure "Mevatron 20," page 9, supra). Such an interlock system may comprise as a measuring device an ionization chamber of a specific structure, see U.S. Pat. No. 4,131,799. The known ionization chamber has two measuring chambers formed by three mutually parallel walls spaced apart by spacer rings. Two of the three walls have single electrodes arranged thereon, whereas a third wall has several mutually insulated electrodes applied on the measuring side of the wall. The mutually insulated electrodes include a central circular disk-shaped electrode and a group of electrode segments arranged in circular fashion around the central electrode. Such a chamber is especially useful for measuring the intensity distribution of an electron beam. It can also be used for measuring X-rays. In the case of a completely homogeneous radiation intensity in the beam cone, the currents in the measuring electrodes of the chamber are equal. If the currents through the individual equal area measuring electrodes of the ring-shaped arrangement differ, an unsymmetrical distribution of the radiation intensity in the beam cone is indicated. However, if the currents through the individual segment measuring electrodes are equal, but different with respect to the current of the center circular disk-shaped measuring electrode, an inhomogeneity of the radiation intensity in the beam cone is indicated. The beam cone is intended to be symmetrical to the axis of symmetry of the ionization chamber, i.e. to the center of the beam cone.
Accordingly it is desirable to provide another interlock system for a particle accelerator, namely an energy interlock system that interlocks the accelerator in case of undesired energy changes of the radiation output. Such an energy interlock system for X-rays is especially important in a linear accelerator which does not dispose of an electron beam bending system (see, for instance, brochure "Mevatron 60," supra). Such an electron bending system, usually a bending magnet system, commonly works as an energy filter or band pass for accelerated electrons (see, for instance, brochure "Mevatron 20," supra). A linear accelerator of the type having no electron beam bending system may experience a drift of signals from its mechanical and electrical components which leads to an X-ray output energy that is too high or too low for the intended irradiation process. Even though a dose monitoring system and a dose rate monitoring system may be working properly, a patient irradiated by the accelerator should be protected from too high or too low X-ray energies.
Assume, for instance, that a linear accelerator disposes of a dose rate control or servo circuit. If for some reason (for instance drift of components or source variation) the radio frequency power supplied by the HF source of the accelerator should increase, while the output dose rate (in r/min) is kept constant by the dose rate control circuit, the energy of the X-rays would also increase. Such an energy increase has to be stopped, as soon as a preset maximum energy level is reached. The same applies to energies which are too low. A decrease in energy should be stopped, as soon as a preset minimum energy level is reached.